Mass ratio and relative driver fatality risk in two-vehicle crashes.

The relative risk, R, of a driver fatality in the lighter of two cars compared to the risk in the heavier is determined as a function of the ratio, mu, of the mass of the heavier to that of the lighter, using Fatal Accident Reporting System data for 1975-1989. In all of many cases investigated, the data fitted well the functional relationship R = A mu u. When the cars differ only in mass, A = 1; if they differ in another dichotomous characteristic, such as old compared to new model years, A estimates the influence of this other characteristic when the masses are equal. The results show that if a driver transfers to a car lighter by 1%, that driver's fatality risk in a two-car crash compared to the risk to the other involved driver increases by between 2.7% and 4.3%, the specific value depending on other factors, such as model year. When one car crashes head-on into the side of another of equal mass, driver fatality risk in the side-impacted car compared to that in the frontally impacted car is 4.5 +/- 0.6 times as great for right-side impacts and 10.1 +/- 1.7 times as great for left-side impacts. Extending the analysis to vehicles other than cars provides empirical evidence that two previously stated "laws" apply systematically over a wide spectrum of vehicles, from mopeds, through motorcycles, small cars, large cars, small trucks to large trucks. These laws are that, when other factors are equal, (1) the lighter the vehicle, the less risk to other road users, and (2) the heavier the vehicle, the less risk to its occupants.